1. Field of the Invention
The present invention relates to an ultrasonic pulse-echo ranging device.
2. Description of the Related Art
Pulse-echo ranging devices, such as time-of-flight ranging devices, are commonly used in level measurement applications for determining the distance to a target object (e.g., a reflective surface of a material in a container) by measuring how long it takes to receive reflected pulses or echoes after transmission of a sequence of energy pulses. Such devices typically use ultrasonic pulses or pulsed radar or microwave signals.
Generally, acoustic pulse-echo ranging systems include a transmitter that generates electrical pulses of high frequency to cause an electro-acoustic transducer to generate acoustic pulses. The same transducer is utilized to receive acoustic energy echoed from the target and to convert such energy into electrical signals that are applied to a receiver. An echo profile may be generated from the received signals by amplifying, filtering and envelope shaping, where at some point an analog-to-digital conversion is performed. Echo pulses are identified in the digital echo profile by a signal processor, and the distance or range of the target is calculated based on the transmit times of the transmitted energy pulses and the identified echo pulses. The calculated distance or range is then transmitted over a communication bus to a process control system using a communications protocol.
From JP 2010-158470 A, an ultrasonic diagnostic apparatus for medical use is known. This conventional device comprises a piezo-electric transducer for emitting ultrasonic waves to a target and receiving echo waves reflected by the target, a transmitter having an output for driving the transducer, a receiver having an input for receiving signals from the transducer, a first transformer having a primary winding coupled to the output of the transmitter and a secondary winding connected to the transducer, and a second transformer having a primary winding coupled to the input of the receiver and a secondary winding connected to the transducer.
Here, the secondary windings of the first and second transformers are connected in parallel.
In level measurement applications, the acoustic wave propagation is affected by attenuation that increases with distance. When the measurement range (distance) is large, the strong attenuation of the echo makes it un-detectable from the ambient noise. Thus, if the conventional device is used for ultrasonic level measurement and an attempt to step up the electrical signal in both directions is made, i.e., from the transmitter to the piezo-electric transducer and from the transducer to the receiver, then the receiver input would be exposed to very high voltage when exciting the transducer to transmit the ultrasonic pulse. This would require protection circuits such as limiters with high series impedance, which would also degrade the received signal. Additionally, the second transformer will have to be dimensioned to handle the amplitude of the transmit pulse. There might be other limitations arising from the fact that the secondary windings of the transformers are connected in parallel with the piezo-electric transducer when the high voltage excitation is applied.